David Cameron is to tell Alex Salmond that he can have a binding referendum on Scottish independence – but only in the next 18 months. Photograph: David Moir/Reuters

There is undoubtedly a logic to the proposal that Westminster should legislate to give the Scottish parliament the right to hold a binding referendum on independence – yes or no. It is to inflict the maximum possible damage on the SNP and its leader Alex Salmond.

The SNP may have won a majority in last May's Scottish parliament election, but this success was not occasioned by any dramatic rise in independence fervour. Most opinion polls suggest independence remains a minority preference. For example, the most recent Scottish Social Attitudes survey put support at just 32%, so it is little wonder that unionists fancy their chances of winning a referendum held now.

However, for some time Salmond has had a rather different kind of ballot in mind. Following a process of consultation with the Scottish public, dubbed the "national conversation", the SNP discovered that, while there were still only limited calls for independence, there was support for a much more powerful Scottish parliament within the framework of the union.

Thus the SNP coined the idea of "devolution max". Under this scenario the Scottish parliament would decide more or less all of Scotland's domestic affairs, leaving just defence and foreign affairs in Westminster's hands. And, while indicating that such an option would be his second preference, Salmond has persistently suggested he was minded to give Scots the chance to vote on that option as well as independence. Opinion polls affirm the idea is popular. A recent Ipsos-Mori poll, for example, found that as many as 68% would vote for it.

However, unionists spot a trap. They fear voters would be confused by being asked to consider how much devolution they want, as well as whether they want independence or not. They suspect the anti-independence argument would be heard less clearly. Indeed, such a referendum would probably create a split in the unionist camp. At present the Liberal Democrats north of the border are developing a proposal for Scottish home rule. In contrast, the new Scottish Conservative leader, Ruth Davidson, has set her face against any further devolution beyond that proposed in the Scotland bill currently before parliament. But above all, unionists calculate that a multi-option referendum would give Salmond two bites at the separation cherry. If voters rejected independence but backed devolution max, he would still have secured a valuable consolation prize. Far from bursting the nationalist bubble, Salmond would appear to have brought Scotland closer to the nationalists' eventual goal of independence.

Yet, in their wish to deny Salmond such a prize, unionists are pursuing a high-risk strategy. Unlike Salmond, none of the unionists parties has an electoral mandate to hold any kind of referendum at all. Consequently, it will be all too easy for him to portray the manoeuvre as an unwarranted interference in Scotland's affairs.

Above all, the unionist strategy assumes that, if they were forced to choose between the current devolution settlement and independence, those who want a more powerful Scottish parliament would opt for the status quo. However, having been denied the chance to vote for what they want, those who want more devolution might wonder whether unionists could be relied upon to deliver any more devolution at all, especially once the threat of independence was removed. If just one in three such voters were to adopt that view then suddenly the outcome of the referendum would look too close to call.

There is a potential alternative unionist strategy – one less concerned to maximise the damage inflicted on Salmond and more concerned to minimise the risk that Scotland might vote for independence after all. It would be for unionists to campaign for a new, more radical devolution settlement themselves. But, at the moment at least, there is apparently little prospect of this path being pursued at all.